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2024
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Diamond types: Type Ia, Type Ib, Type IIa, and Type IIb.
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Based on the nitrogen (N) content in diamonds, they can be classified as Type I diamonds and Type II diamonds.
Based on the nitrogen (N) content in diamonds, they can be classified into Type I and Type II diamonds.
In Type I diamonds, the nitrogen impurity concentration typically exceeds 20 × 10⁻⁶, which is sufficient to be detected by infrared absorption spectroscopy. In contrast, Type II diamonds usually contain nitrogen impurities at levels below 20 × 10⁻⁶, too low to be detected by infrared absorption spectroscopy.
Type I diamonds can be classified into Type Ia and Type Ib based on the aggregation state of nitrogen impurities within their crystal lattice.
Nitrogen atoms in Type Ia diamonds exhibit a relatively complex aggregation state, with nitrogen atoms in the crystal lattice typically neighboring other nitrogen atoms. Type Ia diamonds usually appear yellow or brown, owing to nitrogen impurities that absorb the blue portion of the visible spectrum.
Type I diamonds can be further classified into Type IaA, Type IaB, and Type IaAB (with transitional types between IaA and IaB).
Type IaA diamonds are dominated by the A center (N2, also known as the A‑center), which consists of two adjacent nitrogen atoms substituting for a pair of carbon atoms in the crystal lattice. In Type IaA diamonds, the A centers formed by these two nitrogen atoms do not interact with other nitrogen atoms in the lattice. By contrast, Type IaB diamonds feature a B center (N4), in which four nitrogen atoms surround a lattice vacancy.
In Type Ib diamonds, the nitrogen atoms are arranged in a relatively simple configuration: a single nitrogen atom substitutes for one carbon atom, and the individual nitrogen atoms are not adjacent to one another. This type of impurity nitrogen is referred to as isolated nitrogen, substitutional nitrogen, or a C‑center. Type Ib diamonds typically exhibit yellow or brown hues, with the intensity of color depending on the hydrogen content and its spatial distribution.
Type II diamonds can likewise be further classified into Type IIa and Type IIb.
Type IIa diamonds contain no impurities of nitrogen or boron (B), or only trace amounts that are too low to be detected by infrared absorption spectroscopy. Due to the high density of vacancies in their crystal lattice, Type IIa diamonds exhibit a relatively high refractive index and strong luster. Moreover, because of their nearly pure composition, they typically appear colorless or nearly colorless.
Type IIb diamonds contain no impurity nitrogen but do contain boron atoms. In Type IIb diamonds, boron atoms substitute for carbon atoms in the crystal lattice, resulting in a higher refractive index and birefringence. Moreover, because boron atoms absorb ultraviolet and blue light, Type IIb diamonds typically exhibit a bluish hue.
Diamonds of different types vary in composition, structure, and physical and chemical properties, giving them distinct applications across industries, scientific research, and the jewelry sector. For instance, Type Ia and Type Ib diamonds are widely used in industry—for manufacturing cutting tools, abrasives, and drill bits—while Type IIa and Type IIb diamonds, owing to their unique characteristics and relative rarity, are primarily employed in research and high‑end jewelry.
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